Influences of nutrients on summer algal bloom in the Changjiang plume revealed by high-resolution profiles.

Fine-scale vertical distributions of nitrate and other biogeochemical parameters provide critical insights into nutrient dynamics and their role in fueling algal blooms in coastal waters. In August 2022, we conducted high-resolution vertical nitrate profiling in the Changjiang Estuary using a Submersible Ultraviolet Nitrate Analyzer. In the inner estuary, the mixing of nutrient-rich Changjiang waters elevated nitrate concentrations in the upper layer, forming a positive vertical nitrate gradient. In the plume front, subsurface nitrate maxima were observed, likely driven by upwelling of cold waters and local remineralization. Elevated chlorophyll-a (Chl a) concentrations were observed in both surface and subsurface waters. The horizontal boundary of surface waters with high Chl a (>5 μg L-1) in the plume front coincided with the edge of the nitrate-replete Changjiang plume (salinity < 31 PSU), highlighting the crucial role of plume waters in sustaining algal bloom. The subsurface Chl a maximum was likely influenced by nutrients from the nitrate maximum depth. High surface Chl a were also observed in the upwelling area, extending to depths below the pycnocline (∼10 m), underscoring the importance of upwelled nutrients. To the east of the plume edge, negative vertical nitrate gradients persisted, with deeper pycnocline layers (20-30 m), where limited nutrient availability constrained Chl a accumulation. The high-resolution nitrate data shows illustrate how river plume and upwelled waters collectively shape vertical nitrate profiles and influence algal bloom dynamics in the Changjiang Estuary, represent a crucial direction for advancing our understanding of biogeochemical processes.